The present patent application claims the right of priority under 35 U.S.C. xc2xa7119 (a)-(d) of German Patent Application No. 101 004 04.4, filed Jan. 5, 2001.
The present Application relates to a process for producing polycarbonate by the transesterification and to the recovery of diaryl carbonate produced thereby.
The present Application relates to a process for producing polycarbonate by the transesterification in the melt of diaryl carbonates with dihydroxy aryl compounds. The vapor streams generated in the course of the process contain diaryl carbonate (DAC) that in accordance with the invention is recovered in high quality. The recovered DAC may be reused in the transesterification process. The monohydroxy aryl compounds isolated in high purity in course of the claimed process may be reused either for the production of diaryl carbonate as well as for the production of dihydroxy aryl compounds.
For the production of polycarbonate by transesterification in the melt, dihydroxyaryl compounds are reacted with diaryl carbonate, wherein the monohydroxy aryl component is separated from the diaryl carbonate in the sense of a transesterification reaction. This condensation reaction initially results in the formation of low molecular weight polycarbonate oligomers, which react further to form high molecular weight polycarbonates as the separation of monohydroxy aryl components proceeds. The progress of the reaction can be assisted by the use of suitable catalyst. Moreover, in order to obtain high molecular weights, it is necessary to remove the monohydroxy aryl component which is formed from the reaction space and thus to assist the progress of the reaction. Various measures are implemented industrially in order to efficiently remove the monohydroxy aryl component, such as increasing the temperature of the reaction medium, reducing the pressure in the gas space over the reaction medium, flashing the reaction mixture into a gas space under reduced pressure, introducing inert gases or the vapors of volatile solvents as entraining agents, and the use of special reaction apparatuses which assist the removal of the monohydroxy aryl component by a continuous renewal of the surface, particularly if highly viscous melts are produced. In all the aforementioned embodiments, gaseous vapor streams are produced, which mainly contain the monohydroxy aryl component of the diaryl carbonate. Depending on the type of dihydroxy aryl compound used and on the diaryl carbonate used, the mass of the vapor stream which is obtained can be greater than the mass of the polycarbonate which is obtained. Reuse of the vapor stream obtained is therefore necessary in order to achieve economic production of polycarbonate by the method of transesterification in the melt.
One very important industrial process is the production of high molecular weight polycarbonate from 2,2-bis(4-hydroxyphenyl)-propane bisphenol A; hereinafter called BPA) and diphenyl carbonate hereinafter called DPC). In this case, the aforementioned vapor streams mainly consist of phenol. The phenol which is obtained in the course of this process can be reused in the sense of a recycling operation for producing DPC, which has been published for the first time in Schnell Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, J. Wiley and Sons, Jnc (1964). Further details to the reuse of the phenol as obtained are found for example in WO 93/3084 and LU A 88569. LU A 88564 also describes the use of the phenol obtained for the production of BPA by reaction with acetone. Both for the production of DPC and for the production of BPA, stringent demands are imposed on the purity of the phenol used in order to obtain products of high quality.
U.S. Pat. No. 5,922,827 describes a process for the reuse of the phenol from the transesterification of diphenols and diaryl carbonates. The phenol obtained is used there for producing diaryl oxalate by conversion of dialkyl oxalates by transesterifications into diaryl oxalates which in turn is converted by decarbonylation into diaryl carbonate.
There is no mention of the reuse of DAC in the above patent, however. In U.S. Pat. No. 6,277,945 B1 as well as Japanese Specifications JP 2000053759 A and JP 2000128976 A, phenol is also recovered from the transesterification process and is used for the production of BPA or DPC. In EP A 992 522, a monohydroxyaryl compound is likewise recovered from the transesterification process, but is only used as a solvent/support for the catalyst which is added.
In the practical production of polycarbonates, it has been shown that in addition to the monohydroxy aryl compound of the diaryl carbonate, which is the main component, the vapor streams also contain other components which are either present in the reaction medium directly or which are formed under the prevailing reaction conditions by secondary reactions from components of the reaction medium. Examples of secondary components such as these which can be formed include diaryl carbonates, dihydroxy aryl compounds, catalyst residues or secondary products of spent catalysts, as well as cleavage and rearrangement products of the diaryl carbonate used and of the dihydroxy aryl compounds used, and secondary products thereof. In the case of the industrially important synthesis of polycarbonates from BPA and DPC, the vapor streams contain, in addition to phenol, secondary components such as DPC, BPA, low molecular oligomers from BPA and DPC phenyl salicylate, isopropenylphenol and dimers and oligomers thereof, hydroxyindanes, hydroxychromanes, catalyst residues and secondary products thereof. In particular, the content of DPC in the vapor streams may, under some polycondensation reaction conditions, be greater than 5% by weight in the vapor streams. In the conventional process, this valuable substance is thus lost in not inconsiderable amounts.
Furthermore, additional by-products may also occur in the vapor streams under the prevailing conditions, even after the separation of the polycarbonate, due to cleavage and/or recombination reactions of the aforementioned secondary components. Thus, for example, the reaction of isopropenylphenol and phenol in the vapor streams may result in the formation of BPA.
In principle, separation of the aforementioned secondary components, such as isopropenylphenol, phenyl salicylate or hydroxyindanes, from the polycarbonate melt via the gas phase is desirable, since a higher purity of the polycarbonate obtained would be achieved by the removal of these components. However, the presence of these secondary components in the vapor streams means that the phenol obtained from the condensation of the vapor streams cannot be used directly for producing DPC or BPA or for other chemical reactions, since extremely high purity criteria are generally imposed on the phenol used for reactions such as these. High purity phenol may in fact be obtained from the vapor streams by customary purification methods such as simple distillation or recrystallisation, but valuable substances such as DPC, which are present as secondary components, are not isolated for reuse by such methods.
The above processes therefore have the disadvantage that the DPC which is used in excess is incinerated with the bottom product which remains from the recovery of phenol by distillation. DPC may be present in the bottom product at a content of about 90% by weight, which therefore results in a considerable loss of DPC.
Attempts to obtain highly pure DPC by overhead distillation have generally resulted in failure, on account of the secondary reactions which occur at the high bottom product temperatures which are required.
Thus the object of the present invention, starting from the prior art was to provide a process for producing polycarbonate by means of transesterification in the melt, which makes it possible to recover unreacted diaryl carbonate with high purity from the vapor streams and which at the same time considerably reduces the consumption of raw materials.
Ideally, success should be achieved with a process such as this in separating the vapor streams from the production of polycarbonates by means of transesterification in the melt by a suitable combination of purification operations so that valuable materials (which are generally the monohydroxy aryl compound of the diaryl carbonate and the diaryl carbonate, and in the situation exemplified, namely the production of polycarbonate from BPA and DPC, these are phenol and DPC) are isolated at high yield, wherein the monohydroxy aryl compound which is obtained is of sufficient purity for the production of the corresponding diaryl carbonate and the isolated diaryl carbonate is of sufficient purity to be recycled as a raw material directly to the polycarbonate synthesis comprising transesterification in the melt, without impairing the quality of the polycarbonate. Ideally, secondary components from the process should be removed as completely as possible, and the amount of non-reusable residual substances from the work-up of the vapor, which have to be disposed of, should be  less than 5%, preferably  less than 4%, most preferably  less than 3.5% with respect to the amount introduced into the vapor work-up stage, in order to achieve a low level of unwanted losses of material.